1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly to a waveguide attenuator.
2. Description of the Related Art
A waveguide typically includes a material medium that confines and guides a propagating electromagnetic wave. In the microwave regime, a waveguide normally consists of a hollow metallic conductor, usually rectangular, elliptical, or circular in cross section. This type of waveguide may, under certain conditions, contain a solid or gaseous dielectric material.
In a waveguide or cavity, a “mode” is one of the various possible patterns of propagating or standing electromagnetic fields. Each mode is characterized by frequency, polarization, electric field strength, and magnetic field strength. The electromagnetic field pattern of a mode depends on the frequency, refractive indices or dielectric constants, and waveguide or cavity geometry.
An “evanescent field” in a waveguide is a time-varying field having an amplitude that decreases monotonically as a function of transverse radial distance from the waveguide, but without an accompanying phase shift. The evanescent field is coupled, i.e., bound, to an electromagnetic wave or mode propagating inside the waveguide.
Variable waveguide attenuators are commonly used to attenuate microwave signals propagating within a waveguide, which is a type of transmission line structure commonly used for microwave signals. Waveguides typically consist of a hollow tube made of an electrically conductive material, for example copper, brass, steel, etc. Further, waveguides can be provided in a variety of shapes, but most as previously mentioned often are cylindrical or have a rectangular cross section. In operation, waveguides propagate modes above a certain cutoff frequency.
Waveguide attenuators are available in a variety of arrangements. In one arrangement, the waveguide attenuator consists of three sections of waveguide in tandem: a middle section and two end sections. In each section a resistive film is placed across an inner diameter of the waveguide (in the case of a waveguide having a circular cross section) or across a width of the waveguide (in the case of a waveguide having a rectangular cross section). In either case, the resistive film collinearly extends the length of each waveguide section. The middle section of the waveguide is free to rotate radially with respect to the waveguide end sections. When the resistive film in the three sections are aligned, the E-field of the an applied microwave signal is normal to all films. When this occurs, no current flows in the films and no attenuation occurs. When the center section is rotated at an angle θ with respect to the end section at the input of the waveguide, the E field can be considered to split into two orthogonal components, E sin θ and E cos θ. E sin θ is in the plane of the film and E cos θ is orthogonal to the film. Accordingly, the E sin θ component is absorbed by the film and the E cos θ component is passed unattenuated to the end section at the output of the waveguide. The resistive film in the end section at the output then absorbs the E cos θ sin θ component of the E field and an E cos2 θ component emerges from the waveguide at the same orientation as the original wave. The accuracy of such an attenuator is dependant on the stability of the resistive films. If the resistive films should degrade over time, performance of the waveguide attenuator will be affected. Further, energy reflections and higher-order mode propagation commonly occur in such a waveguide attenuator design.
In another arrangement, a wedge shaped waveguide attenuator having resistive surfaces exists. Because the waveguide attenuator is wedge shaped, the E field again can be considered to split into two orthogonal components at each surface of the wedge, E sin θ and E cos θ. As with the previous example, the E sin θ component of a microwave signal is absorbed by the film. However, The tapered portion of the waveguide attenuator causes energy reflections to occur. Hence, the wedge shaped waveguide attenuator must be long enough to obtain sufficiently low reflection characteristics. Accordingly, this type of waveguide attenuator is limited to use in relatively long waveguides. Thus, a need exists for a waveguide attenuator that provides additional design flexibility and overcomes the limitations described above with respect to existing waveguide attenuators.